Dryland Farming: Strategies to Reduce Heat Stress in Crops and Livestock
When I spent a month in Guatemala in the early 1990s, I realized that the concept of plant guilds was well-known not only in the Sonoran Desert, where I came from, but also in the warmer tropical regions of Central America. I spent a week trying to sleep in the heat of a multistory concrete apartment in a crowded colonia of a Guatemalan city before I made it to a palm-thatched palapa hut in the middle of a coffee plantation.
Although the two houses were less than two miles apart and had the same macroclimate, the microclimate on the coffee plantation became at least a dozen degrees cooler at noon every day. The air conditioning wasn’t built into the wall of the conical hut, but came from the palm thatch on the roof, the coffee trees around the hut, and the dense canopy of trees above us.
The influence of plant care guilds
The cultivated tree that provided us with relief from the heat is known throughout Latin America as mom cocoamother of cocoa. It is a nitrogen-fixing legume known to scientists as Gliricidia sepium. It may be the world’s most widespread woody tropical plant today, providing “shade for cocoa, coffee and other shade-loving plants.”
Although scientists have supported the spread of madre de cacao as a coffee care plant in many parts of the Americas, the Caribbean, Africa, Asia, and the Pacific Islands, its first operate as a buffer against heat stress probably began in the Mayan-dominated regions of Central America, where madre de cacao was first recruited to provide shade for tender teenage cacao plants. This must have happened in old times, because madre de cacao is associated with both chocolate and tree monkeys in the old Mayan epic Popul Vuh.
As historians of the Maya agricultural landscape write, cocoa cultivation became dependent on madre de cacao trees “because it requires a finely tuned ecosystem to survive: it is sensitive to wind, sun, drought, and is dependent on nitrogen.” If it were not for lofty, shady, nitrogen-fixing windbreaks and helper plants such as the madre de cacao and its close relative the cocoa tree (balam-té, the guardian tree), we may never have been able to enjoy the pleasures of eating chocolate or drinking sizzling cocoa.
Variety of care plants
Mesquite serves as a shelter tree for herbs at Rancho el Peñasco Eco-Lodge in Sonora, Mexico.
Although I was impressed by the operate of nurse trees as a thermal buffer for heat-sensitive crops such as coffee and cocoa in the Central American tropics, it seems that the diversity and importance of care plants is much more evident in the deserts of North and South AmericaIn fact, there are dozens of species of trees that desert inhabitants call nodrizas or madrinas because they are absolutely indispensable for the germination and survival of much of the edible flora that grows in the sizzling, droughty climate.
How much difference can a care plant cover make in protecting the understory from damaging heat and harmful sunlight? Working with my Mexican colleague and former student Humberto Suzán, we once collected a year and a half of data on temperatures in microenvironments under guardian trees in the Sonoran Desert.
Temperature testing to reduce heat stress
Previous research — conducted long before climate change became so apparent — suggested that the dense shade of a mature nurse tree could potentially lower the maximum soil temperature beneath it by 20 ̊F (11 ̊C) degrees on a summer day and raise the minimum temperature by 5 ̊F (3 ̊C) on a winter day. We decided to verify this research by measuring soil temperature both in the understory and outside the understory of desert ironwood.covered with a dense, evergreen canopy, providing continuous shade all year round.
Compared to the 115.2 ̊F (46.2 ̊C) of desert soil fully exposed to the sun at noon, the soil temperature under the dense shade on the north side of the ironwood canopy was only 95.8 ̊F (35.4 ̊C), almost 20 ̊F degrees cooler! Even more remarkably, the temperature of the cactus stems under the same ironwood was only 94.8 ̊F (34.8 ̊C), within the range in which even pinto beans could grow and flourish. The microclimate under the nearby mesquite trees was almost as well buffered as under the ironwoods, with their temperatures hovering around 98.3 ̊F (36.8 ̊C) at noon in July.